1. Field of the Invention
The present invention relates to a method for manufacturing a dynamic random access memory, and more preferably, to a method for manufacturing a dynamic random access memory having a step of processing in an atmosphere of a hydrogen gas.
2. Related Art
Thin film polysilicon transistors (hereinafter referred to as “TFT” from the acronym of “thin film transistor”) are often used in conventional semiconductor devices.
When such a TFT is used in a memory channel having a P channel MOS thin film polysilicon transistor stacked on N channel MOS field effect transistor, a semiconductor device on which the TFT is mounted spends too much standby current, which presents a problem.
This problem is known as caused by a trap level due to defects in polysilicon particle boundary or particles contained in the TFT.
As this trap level is formed by dangling bond contained in the polysilicon, decrease in the dangling bond effectively reduces the standby current. The dangling bond is terminated by hydrogen. In view of this, there has been proposed a method of terminating dangling bond contained in the polysilicon with use of hydrogen contained in plasma nitride film used in the polysilicon.
Specifically, in order to prevent diffusion of unnecessary OH groups inside the polysilicon when forming an oxide film in the TFT by wet reflow, a silicon nitride film is sometimes formed on the polysilicon as an OH group stopper.
Presence of this silicon nitride film prevents hydrogen contained in the plasma nitride film provided above the silicon nitride film from reaching a channel portion of the TFT formed of polysilicon below the silicon nitride film.
In order to solve this problem, there is proposed a method of manufacturing a semiconductor device having a step of forming a hole in the silicon nitride film (see Japanese patent application publication No. H5-129333).
There is proposed another method of manufacturing a semiconductor device including a step of radiating a semiconductor substrate, which has a semiconductor layer of polysilicon and the like and an insulating film formed on the semiconductor layer, with light for splitting a hydrogen gas into hydrogen atoms in an atmosphere where the hydrogen gas is contained (see Japanese patent application publication No. 2005-217244).
On the other hand, with recent advance of technology such as down sizing and weight reduction of electronic devices, a packing density per unit area of a dynamic random access memory tends to exhibit a significant increase. This increase of the packing density tends to increase a defect rate of the dynamic random access memory.